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Facies and geometry of drowning steps in a Miocene carbonate platform (Maldives)

J. Reolid, C. Betzler, J.C. Braga, T. Lüdmann, A. Ling, G.P. Eberli
Palaeogeography, palaeoclimatology, palaeoecology 2020 v.538 pp. 109455
Miocene epoch, Retaria, Rhodophyta, algae, benthic organisms, carbonates, fossils, geophysics, monsoon season, sea level, sediments, Maldives
The study of the Miocene carbonate platform of the Maldives allows understanding the controlling factors triggering the stepwise drowning of carbonate platforms. This research presents high-resolution seismic profiles and sediment cores retrieved from Sites U1465, U1469, and U1470 drilled during International Ocean Discovery Program (IODP) Expedition 359. A microfacies analysis of the platform deposits was conducted and correlated with a sequence stratigraphic analysis of seismic reflection data. The Neogene evolution of the Maldives encompasses a series of drowning steps. Two of these steps are compared with regard to the carbonate factory type to unravel if fossil assemblage surviving the first drowning step are different from younger assemblages. In both cases the shallow water carbonate facies are formed by coral-coralline algal boundstone and foralgal floatstone that change basinwards into slope facies dominated by large benthic foraminifera grainstone to packstone, locally floatstone and rudstone. The red algae and large benthic foraminifera assemblages are similar before and after the first drowning step. After this first drowning, the carbonate platform developed under enhanced bottom current conditions, as deduced by the coeval drift deposits. The carbonate platform and the drift deposits were balanced for circa 2 Myr, until a global sea-level fall at 10.6 Ma and later reflooding of the platform, which destroyed the equilibrium between both depositional systems, favouring the current-controlled deposits. The drowning of the Miocene carbonate platform of the Maldives was the result of the combined effect of the fluctuating sea-level during the Neogene and the intensification of oceanic currents due to the invigoration of the South Asian Monsoon during the middle Miocene.